The lipids of animal and vegetable origin (oils and fats, that is, triglycerides) are the sources of the so-called biodiesel (BD) fuel, which, as a result of its energy content approaching the combustion heat of hydrocarbons, is becoming a more and more important factor in traffic, as an acknowledged and supported alternative combustible. This way for example, according to the EU directives, by 2010 it must be a 5.75% component of the fuel of diesel-operated vehicles.
The triglycerides are the triesters of glycerol, a trivalent alcohol, formed with different fatty acids. Most of the fatty acids are C16-C22 acids, but in some triglycerides shorter (C4-C15) and/or longer (C23-C28) fatty acid chains may also occur. The fatty acids esterifying the glycerol are mostly saturated in the fats (i.e. stearic acid, palmitic acid), in the oils the different acids may contain 1-3 unsaturations (oleic acid, linolenic acid), and this is the cause of the lower melting point of the latter lipids. (According to convention, triglycerides still liquid at 25° C. are called oils). As a result of their energy content animal fats and vegetable oils would be applicable for the propulsion of diesel-operated engines without any chemical transformation. But their molecular masses and viscosities are too high (850-1000; >30 cSt at 40° C.), this way they cannot be fed or atomised directly into the modern engines in the proper amount and form. That is why their molecular masses must be reduced to the atomisation limit value: the substances with lower molecular masses are thinner, their pour point is lower, this way they can be used as fuel too. This effort led to the “biodiesel” (BD, mixture of fatty acid methyl esters): the 3 equivalent fatty-acid methyl esters and the 1 equivalent glycerol are produced by the “fragmentation” (methanolic transesterification, or more correctly: alcoholysis) of the triglycerides (their molecular masses being 290-310, their viscosities 4-5 cSt):

The glycerol obtained in 10-12% in itself is unsuitable for fuel purposes; its combustion heat is low, and its viscosity is orders of magnitude higher than that of the original triglyceride. Its appearance as an industrial by-product (and in a heavily contaminated form) had serious consequences: glycerol lakes and sewage waters contaminating the environment. Their elimination is a serious environmental burden all over the world. By recognizing this, in order to replace the current manufacturing of biodiesel, research was conducted by us for a new technology and a new type of fuel, the starting materials of which are also natural triglycerides.
Alcoholysis of the triglycerides can be executed by acid- or base-catalysis. Because of the higher rate of reaction the latter one is more widespread. The operation is relatively quick (1-3 hours, 30-70° C.), it can be a continuous or batch procedure. Because of the equilibrium reaction, methanol is used at least in twofold molar excess, at the end of the reaction the excess is evaporated and recirculated. Following separation of the lower glycerol phase and washing with water, the ester mixture is vacuum distilled, and the biodiesel is obtained with 98-99% yield, and this is mixed with hydrocarbon based diesel oil (for example JP 7197047, DE 3515403). There are procedures with high energy demand, using extreme reaction conditions (260° C., 10 MPa) (FR 2,752,242).
Alcoholysis is carried out with refined (purified, freed from organic polymer and wax) lipids. But at the same time the free fatty acid content of the triglycerides occurs as a problem, which in case of animal fats and/or used household oils can be very high (15-20%). In this case the fatty acids are generally esterified with acid catalysis in the first step (because the free fatty acids generate soaps with base catalysts, which results is emulsions, and these are difficult to process), the acidic catalyst is washed out, and alcoholysis of the triglyceride is carried out in basic conditions (WO0112581) [Excellent review of the current status of the biodiesel technique can be found in the literature: E. Lotero et al., Catalysis, 19, 41-83 (2006); which can be read on the Internet too].
In recent years many patents have claimed the different embodiments of this “classic” biodiesel manufacturing. For example in U.S. Pat. No. 5,730,029 the mixture of the fatty acid-methyl esters are prepared from the natural triglyceride starting material of indicated origin and composition, according to the reaction of column 6. In Patent No. EP 626,442 also the biodiesel is prepared by methanolic transesterification, according to the “traditional” reaction scheme found on page 5. In Application No. WO 2005/028597 the fatty acid methyl esters are prepared also from triglycerides, as starting material, i.e. according to the “glycerol” route. In US Application No. 060199970 there is a proposal for the utilization of the glycerol side product prepared by the conventional route, according to which it is added to the mixture of the methyl esters prepared as above, in the form of glycerol-acetals. In US Application No. 040108219 mixture of low viscosity unsaturated fatty acid methyl esters and glycerol are produced from triglycerides with high degree of unsaturation by methanolic transesterification. Hungarian patent no. HU 207117 also prepares “fatty acid ester mixtures applicable as fuel” according to the classic “glycerol” technology. A great disadvantage of all the patents mentioned above is the generation of the highly contaminated glycerol side product and the high amounts of washing waste water.
Although glycerol is a valuable starting material for the chemical industry and it can be used in itself in many application fields, but as a result of the running up of biodiesel production in the past years, huge amounts of unmarketable stocks accumulated from this material, and that is why the “glycerol chemistry”—i.e. investigation of new application fields—is in the centre of interest. The situation is complicated by the fact that in the different technologies crude glycerol is produced, characterized by high water content (8-15%), methanol content (1-10%), the presence of mono- and diglycerides, etc. Dehydration of glycerol is an energy consuming process in itself, which, combined with the purification steps relating to the other contaminants, makes this potential “starting material” extremely expensive.
The most plausible utilization of the glycerol prepared this way—just because of the generation of the huge amounts—would be its “feedback” as fuel or fuel additive, after the transformation to suitable derivatives. And indeed, numerous patent applications published lately offer solution to this. For example according to U.S. Pat. No. 6,890,364 acetals, ketals, etc. are prepared from the suitably prepared (purified, dehydrated) glycerol, and their physicochemical properties allow the application of a small portion of the glycerol as fuel, or as additive of biodiesels. According to FR patent No. 2,866,653 ether derivatives are prepared from the glycerol of biodiesel production with olefins, which are added to the fuel, improving with this the emission values. But the biodiesels may not be diluted too much with these derivatives, because of their low combustion heat. And since the biodiesels themselves are—within a foreseeable period of time—only additives (in 5-20%) of the petrodiesel, it is obvious that the utilization of the waste glycerol in this form can only be limited.
On the basis of what was said above it is obvious that the costs of the current biodiesel production cannot be reduced significantly, that is, the competitiveness of the fatty acid-methyl esters cannot be increased, even with the distinctive tax policy. Manufacturing is already optimized and automated, the price of vegetable oils will not decrease because of the growing demand, and the possible price reduction of methanol, because of its small consumption norms, cannot significantly influence biodiesel prices. The glycerol side product is rather a burden than a cost reduction factor for biodiesel factories. The consequence of this is that the direct utilization of triglycerides as fuel is of continuing interest (see for example Hungarian Patent No. 208 994).
In connection with this two publications may be mentioned: On the basis of their systematic studies R. O. Dunn and M. O. Bagby, two well known experts of the field propose many different solutions for viscosity reduction, allowing the direct application of vegetable oils (addition of co-solvents and solubilisation to inhibit later phase-separations, and at the same time for the permanent decrease of pour point), for example by the formation of soybean oil, 2-octanol and ethanol three phase systems [R. O. Dunn and M. O. Bagby: Journal of the American Oil Chemists' Society, 77(No. 12), 1315-1323 (December, 2000)]. In the July issue of the same journal R. O. Dunn proposes the introduction of a new multicomponent system [Journal of the American Oil Chemists' Society, 79(No. 7), 709-715 (July, 2002)].